Conventional mobile phone apparatuses often use a temperature compensated quartz crystal oscillator (VCTCXO) to control the master clock frequency needed for demodulating the received signal, according to base-station synchronization signal. However, such a temperature compensated quartz crystal oscillator is expensive, big in size and can not be made on an integrated semiconductor IC.
According to another implementation, a mobile phone apparatus uses an uncompensated quartz crystal oscillator that is frequency controlled depending on the carrier frequency used by a base station and derived from the received signal. In this case there are problems to keep precise clock frequency accuracy when base station synchronization gets lost, i.e. during a temporary reception gap period. Such reception gaps occur whenever a mobile phone user leaves the coverage area of a base station so that connection between the mobile phone and the base station is lost suddenly, e.g. when the user is moving through a tunnel etc.
A receiver known from U.S. Pat. No. 4,947,409, e.g. a receiver of a mobile telephone comprises a high-frequency stage and an intermediate-frequency stage for mixing down a signal received by an antenna to obtain two quadrature signals, i.e. an inphase signal and a quadrature phase signal. The mixing frequencies are supplied to the high-frequency and an intermediate-frequency stages from a local oscillator. This oscillator is controlled by a control processor in accordance with a frequency error signal derived from the two quadrature signals so that it operates in synchronism with the transmitting station.
Such a prior art mobile phone apparatus is suffering from poor accuracy. When such mobile phone loses synchronous operation to the base station it can not immediately get the link back when coverage reappears again, i.e. when the mobile phone user enters the coverage area of a base station again. Instead it takes some unacceptable long time to become synchronized again and to continue mobile link services.
In U.S. Pat. No. 5,790,604 a method and an apparatus for automatic frequency correction acquisition is described that is used in a subscriber receiver of a wired TV and telephone system. This apparatus comprises an automatic frequency correction loop that can be operated in two different modes, i.e. in an acquisition mode and in a tracking mode. The former provides an estimate of the frequency offset relatively rapidly, but in the latter mode the automatic frequency correction loop follows changes in the frequency offset more accurately.
In operation, when the receiver first attempts to acquire a new modulated carrier, either upon power up or upon a change of the carriers at the transmitter, the automatic frequency correction loop is switch into the acquisition mode. When the frequency offset obtained by this mode becomes more accurate, the automatic frequency correction loop is switched into the tracking mode to further increase the accuracy of the frequency offset estimate. However, U.S. Pat. No. 5,790,604 is silent about determining the frequency offset in case of a temporary reception gap.